The focus of this work has been to understand the molecular details that control the interaction of MHC molecules with NK receptors, T cell receptors, and with coreceptors such as CD8. These studies are dependent upon structural, functional, and biophysical analysis of the interaction of the molecules in question, and attempts are made to correlate binding properties with function and structure. Progress in the past year has been particularly fruitful with respect to: 1) binding studies of Ly49C with H-2Kb and with H-2Kb complexed with variant peptides and mutant beta 2-microglobulin; 2) X-ray structure determination of the Ly49C/H-2Kb complex and of a high affinity mutant of Ly49C complexed with H-2Kb. These studies have resolved a major controversy concerning the site of interaction of the murine lectin-like NK receptors with MHC-I and raise additional questions as to how receptors with limited, but degenerate specificity function. Studies of the binding interactions of viral MHC-like proteins with various C-type lectin like NK receptors shed light on the role of NK immunity to viral infection. Progress in the expression and characterization of a set of cytomegalovirus encoded MHC-I like proteins has been made.